Automatic gain control circuit



Feb. 8, 1938. J. F. DREYER, JR

AUTOMATIC GAIN CONTROL CIRCUIT Filed July 30, 1956 DETECTOR TUAF.

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Patented Feb. 8, 1938 UNITED STATES PATENT OFFICE John F. Dreyer, Jr.,Brooklyn, N. Y., assignor to Radio Corporation of America, a co p rationof Delaware Application July 30,

'7 Claims.

My present invention relates generally to gain control circuits forsignal transmission systems, and more particularly to automatic volumecontrol arrangements for radio receivers of the type employing signaltransmission tubes of the electron beam type having negative mutualconductance characteristics.

In my copending application Serial No. 746,495, filed October 2, 1934, Ihave disclosed automatic volume control arrangements for broadcastreceiver constructions, wherein the signal transmission tubes arecontrolled in gain by having them decrease in mutual conductance when anelectrode of the controlling signal rectifier varies in potential in apositive sense. By virtue of such connections the automatic volumecontrol connection may be made to an electrode of the control rectifierwhich varies in potential in a positive direction as the received signalamplitude increases. Specifically such operation is secured in thearrangement of the aforesaid application by utilizing in the controlledtransmission stage a tube provided with a cathode, a signal grid, anoutput electrode, an auxiliary positive electrode, and a special gaincontrol electrode which is disposed between the output electrode and theauxiliary electrode, the output and auxiliary electrodes being positivewith respect to the gain control electrode, and the control electrodebeing geometrically related to the auxiliary and output electrodes sothat the mutual conductance between the output electrode and signal gridis reduced, and the transmission efficiency of the stage isautomatically decreased, :15 as the received signal amplitude increases.The

specific types of tubes shown in the aforesaid application are of thenormal construction wherein the various electrodes are disposed inconcentric manner about the cathode.

It may be stated that it is one of the main objects of my presentinvention to secure the same type of automatic volume control action aswas secured with the arrangement of my copending application, and yetemploy for the controlled signal transmission tube an electron dischargetube of the beam type, the controlled network being essentiallycharacterized by the fact that the signals are impressed upon a controlgrid through which passes an electron beam, focusing electrodes beingemployed to cause the. beam to fall on an output electrode connected tothe signal output circuit feeding into a signal rectifier, and therebeing disposed within the tube a waste, or deflecting electrode which isunder the control of the signal rectifier so as to divert the electronbeam from the output electrode when the signal amplitude increases.

Still other objects of the invention are to improve generally automaticvolume control systems for radio receivers, and more especially to 1936,Serial No. 93,361

provide an automatic volume control arrangement for a receiver of thetype employing an electron beam tube, having a negative mutualconductance characteristic, in a signal amplifier stage, and whichcontrol arrangement is not only reliable, free from distortion andcross-talk, and is eflicient in operation, but is readily andeconomically assembled in a receiver.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have indicateddiagrammatically a circuit organization whereby my invention may becarried into effect.

Referring now to the accompanying drawing, there is shown in schematicmanner the circuit arrangement of that portion of the radio receiverembodying my present invention. In

order to preserve simplicity of disclosure the invention is shownincorporated in the IF amplifier of a superheterodyne receiver. Thoseskilled in the art will readily understand that the tube I, whose signalinput circuit 2 is fixedly tuned to the operating IF, is preceded by theusual networks employed in a superheterodyne receiver. For example, suchnetworks comprise a signal collector followed by one, or more, stages oftunable radio frequency amplifier, the amplified sigr als being fed to afirst detector. The output of the first detector is fed to the inputcircuit 2, and it will be understood that there is impressed upon thefirst detector locally produced oscillations from a local oscillator.The usual variable tuning condensers are employed in the localoscillator, first detector and radio frequency amplifiers, and therotors of these condensers will be arranged for uni-control adjustment.The operating IF may have a value of from '75 to 465 kc., and it should,also, be understood that the circuit 2 may be preceded by one, or more,stages of IF amplification.

The output circuit of the IF amplifier tube l is designated by thenumeral 3, and the circuit is resonated to the operating IF, the circuit3, in turn, is coupled to the tuned input circuit 4 of the seconddetector diode 5. The circuit 4 is tuned to the operating IF, and theanode of diode 5 is connected to the high alternating potential side ofthe circuit 4. The cathode of the diode 5 is connected to the lowalternating potential side of the circuit 4 through the diode loadresistor 6; the latter having connected in shunt therewith the IFby-pass condenser I. The audio component of the rectified IF currentflowing through resistor 6 is impressed upon one, or more, stages ofaudio frequency amplification through a path which includes theadjustable tap 8- and the audio coupling condenser 9. The direct currentcomponent of the IF current flowing through resistor 6 is employed forautomatic volume control (AVC hereinafter), and the numeral l0 denotesthe AVG connections. While the AVG connections are shown to theamplifier I, it will be understood that such connections can be made toone, or more, of the preceding signal transmission tubes, and the usualfilter net-work II is employed to suppress the pulsatr ing components inthe AVG voltage. Of course, the audio network can be followed by anydesired type of reproducer.

The specific connections to the tube 1 will now be considered, and it isfirst pointed out that the tube I is of the .electron beam type. Thetube, in general, comprises a cathode l2, which may be of the indirectlyheated type, a control grid l3 surrounding the cathode I2. A focusingelement I3 may be employed adjacent the grid l3, if desired. The tubeenvelope may be of glass or metal, and between the grid l3 and one sideof the envelope there are disposed the electrodes I 5, l6 whose planesare parallel to one another. Between the grid l3 and the electrodes l5and I6 are disposed electrodes I I and [8. These electrodes l1 and I8are parallel to one another, but the planes thereof are perpendicular tothe planes of electrodes I5 and I 6. It will be observed that theelectron beam projected from cathode l2, and passing through grid I3, isrequired to pass between electrodes l1 and I8, before impinging uponelectrodes IE or IS.

The dotted line 20 denotes an electron beam; it will be noted that thebeam is caused to impinge upon the electrode l5 by virtue of thepositive potential on electrode l8 as well as by virtue of the positivepotential of electrode l5. ,It will,

also, be noted that the electrode I5 is spaced inwardly from theelectrode "5, and is in overlapping relation thereto. A tube of the typejust described may be constructed in many difierent forms. Since thepresent invention is not directly concerned with the specificconstruction of tube I, but rather in its utilization in an automaticvolume control circuit, reference is made to the pendingapplicationSerial No. 29,808 of H. C. Thompson, filed July 5, 1935 whichdiscloses various features of construction which may be employed for thetube I.

In general, the aforesaid Thompson application shows a tube havingnegative mutual conductance characteristics due to the construction ofthe tube, as well as the manner in which the electrodes may beenergized. The connections from the signal circuits to the electrodes oftube I will now be described. The direct current energization potentialsare derived from a voltage source 2|, and the latter may be any wellknown type of direct current voltage supply bleeder resistor. Thebleeder resistor 2| has one side thereof grounded and may comprise thenegative terminal, whereas the other side of the resistor may beestablished at a value of approximately 250 volts. The cathode I2 isconnected to the grounded side of bleeder 2| through the usual signalgrid biasing network 22, and the low alternating potential side of theinput circuit'2 is grounded. The signal grid l3, being connected to thehigh alternating potential of input circuit 2, is at a negative directcurrent potential with respect to cathode l2 which is equal to thevoltage drop across the grid bias resistor 22.

The electrode I5 is connected by lead 24 to the high alternatingpotential side of the IF output circuit 3. Accordingly, it will be seenthat electrode I5 is the working, or output, electrode of the network.The electrode I8 is connected by lead 25 to a point 26 on bleeder 2|;point 26 being at a positive potential with respect to ground, but beingless positive than the point 21 to which the low alternating potentialside of the signal output circuit 3 is connected. The electrode I 6 isconnected by lead 29 to the positive potential point 21. It will benoted that electrode l6 connected to the low alternating potential sideof the IF output circuit 3, and hence functions as a waste, ordiversion, electrode. In other words, when the electron beam 20 isshifted, or diverted, from electrode l5 and strikes electrode IS, theelectron flow through output circuit 3 is decreased, such electronsbeing diverted through the path including lead 29.

The electrode l1 controls the electron beam, and this is accomplished byconnecting electrode I 1 to the AVG connection l0 through lead 3|. TheAVC lead I0 is connected to the cathode side of the'load resistor 6, andthe anode side of the resistor is connected by lead 32 to the point 33of bleeder 2|. The point 33 is at a direct current potential which has avalue lower than the potential value of point 26.

In considering the operation of the arrangment described heretofore, itis first pointed out that with no signals impressed on input circuit 2the voltage drop across resistor 6 will be substantially zero. Thismeans that the gain control electrode H has a potential determined bypoint 33. The electron beam 20 will be in the position shown in thedrawing since the electrode l8 attracts the electron beam in itsdirection by virtue of the positive potential impressed on it from point26. When signals are impressed on input circuit 2 there is a voltagedrop across resistor 6, and the cathode side of the resistor 6 becomesincreasingly positive. This causes the electrode I! to increase inpositive potential, and shift the position of the beam 20 so that it nowfalls upon the waste electrode l8. This means that the signal currentoutput in circuit 3 will be a maximum when the received signal carrieramplitude is a minimum. As the received signal carrier amplitudeincreases in value, the positive potential of electrode I! willincrease, and cause the diversion of the electron beam from the outputelectrode to the waste electrode. This causes a reduction in the signalcurrent output in circuit 3; which means that the signal energy outputof the IF amplifier I decreases with increasing signal amplitude. Inthis way the IF carrier amplitude at the input circuit 4 of the seconddetector is maintained 'substantially uniform regardless of fadingeffects. It will be appreciated that the reduction in gain of thecontrolled signal transmission network is accomplished even though thecontrolling electrode is increasing in positive direct currentpotential.

The output electrode l5 overlaps the waste electrode l6 slightly inorder to make the operation of the beam tube more definite. If therewere a. Space between the adjacent edges of electrodes l5-l6, thecurrent change in the tube would not be as sharp and definite as it iswhen the slight overlapping exists. The electrode l8 positions the beam20 on the output electrode l5 and at the lower edge thereof when thereis no signal being received. Further, the electrode l8 draws the beamback onto the output electrode l5 whenever the signals entirelydisappear. The potential adjusting means 40 for the focusing element I3is well known as to function, and need not be further described.

While I have indicated and described a system for carrying my inventioninto effect, it will be apparent to one skilled in the art that myinvention is by no means limited to the particular organization shownand described, but that many modifications may be made without departingfrom the scope of my invention, as set forth in the appended claims.

What is claimed is:

1. In combination with an electron beam tube of the type including anemission element, an output electrode arranged to receive an electronbeam from the emission element, and a control electrode, said tubehaving a negative mutual conductance characteristic, means forimpressing signals upon the control electrode of the tube, an outputcircuit connected to the output electrode of the tube, a signalrectifier coupled to the output circuit, at least one electron wasteelectrode disposed within the tube, a beam deflection electrode disposedwithin the tube, and a direct current voltage connection from the beamdeflection electrode to the signal rectifier such that the electron beamis deflected from the output electrode to said waste electrode as thesignal amplitude increases.

2. In combination with an electron beam tube of the type including anemission element, an output electrode arranged to receive an electronbeam from the emission element, and a control electrode, said tubehaving a negative mutual conductance characteristic, means forimpressing signals upon the control electrode of the tube, an outputcircuit connected to the output electrode of the tube, a signalrectifier coupled to the output circuit, at least one electron wasteelectrode disposed within the tube, a beam deflection electrode disposedwithin the tube, a direct current voltage connection from the beamdeflection electrode to the signal rectifier such that the electron beamis deflected from the output electrode to said waste electrode as thesignal amplitude increases, and an electrode within the tube energizedto position the beam on said output electrode in the absence of receivedsignals.

3. In combination with an electron beam tube of the type including anemission element, an output electrode arranged to receive an electronbeam from the emission element, and a control electrode, said tubehaving a negative mutual conductance characteristic, means forimpressing signals upon the control electrode of the tube, an outputcircuit connected to the output electrode of the tube, a signalrectifier coupled to the output circuit, at least one electron wasteelectrode disposed within the tube, a beam deflection electrode disposedwithin the tube, a direct current voltage connection from the beamdeflection electrode to the signal rectifier such that the electron beamis deflected from the output electrode to said wast-e electrode as thesignal amplitude increases, and a second deflection electrode in saidtube, means for energizing the said second deflection electrode to drawthe beam back to the output electrode in the absence of signals.

4. The combination with an electron beam discharge tube of the typecomprising a cathode, an output electrode, a waste electrode adjacentsaid output electrode, a control grid surrounding said cathode formodulating and concentrating into a beam the electron discharge fromsaid cathode to said output electrode, and a deflecting electrodebetween said cathode and said output and waste electrodes and beside thepath of the electron beam, of a signal input circuit including saidcontrol grid, a signal output circuit connected to said outputelectrode, a signal rectifier connected to said output circuit forproducing a uni-directional positive potential dependent on the signalamplitude, and a direct current voltage connection between saidrectifier and said deflecting electrode for rendering said deflectingelectrode increasingly positive to pull the electron beam from saidoutput electrode to the waste electrode as the signals increase inamplitude.

5. The combination with an electron beam discharge tube of the typecomprising a cathode, an output electrode, a waste electrode adjacentsaid output electrode, a control grid surrounding said cathode formodulating and concentrating into a beam the electron discharge fromsaid cathode to said output electrode, a deflecting electrode betweensaid cathode and said output and waste electrodes and beside the path ofthe electron beam, of a signal input circuit including said controlgrid, a signal output circuit connected to said output electrode, asignal rectifier connected to said output circuit for producing auni-directional positive potential dependent on the signal amplitude,and a direct current voltage connection between said rectifier and saiddeflecting electrode for rendering said deflecting electrodeincreasingly positive to pull the electron beam from said outputelectrode to the waste electrode as the signals increase in amplitude,and an electrode in said tube, adjacent said deflecting electrode,energized to attract the beam back to the output electrode as thesignals decrease in amplitude.

6. In combination with an electron beam tube of the type including anemission element, an output electrode arranged to receive an electronbeam from the emission element, and a control electrode, means forimpressing signals upon the control electrode of the tube, an outputcircuit connected to the output electrode of the tube, a signalrectifier connected to be responsive in direct current voltage output tothe signal amplitude, a beam deflection electrode disposed within thetube, and a direct current voltage connection from the beam deflectionelectrode to the signal rectifier such that the electron beam isdeflected from the output electrode as the signal amplitude increases,

7. In combination with an electron beam tube of the type including, theemission element, an output electrode arranged to receive an electronbeam from the emission element, and a control electrode, means forimpressing signals upon the control electrode of the tube, an outputcircuit connected to the output electrode of the tube, a signalrectifier, a beam deflection electrode disposed within the tube, adirect current voltage connection from the beam deflection electrode tothe signal rectifier such that the electron beam is deflected from theoutput electrode as the signal amplitude increases, and means toposition the beam on said output electrode in the absence of receivedsignals.

JOHN F. DREYER, JR.

